Response to "Carotid Flow Augmentation as a Risk for Small Vessel Disease of the Brain"

To the Editor: The carotid flow augmentation index (FAI) as measured in our laboratory had a within-measurement reproducibility of 6.4 ± 2.9% (coefficient of variation) and a intermeasurement reproducibility of 9.0 ± 5.5%.1 We observed that FAI was significantly related to the age of the subject and to radial augmentation index (AI) (Supplementary Table S1)1, in agreement with the findings by Hirata et al.2 In one group of participants (n = 349), carotid tonometry was applied to measure carotid pressure AI (PAI). Carotid FAI showed a significant positive association with carotid PAI (r = 0.50, P < 0.0001). The FAI values in our study showed features similar to those reported by Hirata et al. However, the association between carotid FAI and PAI in our study was smaller than the one (r = 0.913) reported by Hirata et al.;2 this discrepancy is possibly attributable to the effects of “noise”. If there is strong association between PAI and FAI, as reported by Hirata et al., PAI can be used as a surrogate index of FAI. Measurement of PAI in either the carotid or the radial artery, is an established procedure in clinical practice. In our study, there was no significant difference in carotid PAI between subjects with lacunar infarcts (n = 41) and those without (n = 308) (18.2 ± 13.0 vs. 21.0 ± 13.5%, respectively, P = 0.21). A similar finding was also reported by another group of researchers.3 They evaluated white matter hyperintensities (WMHs), a manifestation of brain small-vessel disease, in 179 patients. They observed that the occurrence of WMHs was significantly associated with central systolic blood pressure (SBP) measured from the radial waveform, but not with PAI. Their findings together with our results indicate that central BP is more strongly associated with brain small-vessel disease than PAI, which should be highly significantly related to FAI.